docs: def user(self, user: str) -> "ChildHTTPAPI":
          """
          Get a child HTTPAPI instance.
  
          Args:
              user: The Matrix ID of the user whose API to get.
  
          Returns:
              A HTTPAPI instance that always uses the given Matrix ID.
          """
          if self.is_real_user:
              raise ValueError("Can't get child of real user")
  
          try:
              return self.children[user]
          except KeyError:
              child = ChildHTTPAPI(user, self)
              self.children[user] = child
              return child